In practical fuel cell systems, the output of a single fuel cell is typically less than one volt, so connecting multiple cells in series is required to achieve useful operating voltages. Typically, a plurality of fuel cells are mechanically stacked up in a “stack” and are electrically connected in series from the anode of one cell to the cathode of an adjacent cell via intermediate stack elements known in the art as “interconnects”.
In an SOFC stack, the interconnect must perform a variety of functions, including:                Low resistance electrical contact at operating temperatures of 500° C. to 1000° C., operating for tens of thousands of hours;        Robust connection that endures vibration, shock, and thermal cycling;        Separator plate, in the case of a planar SOFC stack, to prevent intermixing of the anode fuel gas stream for one cell with the cathode air stream of the adjoining cell; current must pass through the separator plate;        Must be porous or have passages that allow access of the anode fuel gas and the cathode air to their respective surfaces of the cell;        In some cases, the interconnect also provides mechanical support to the cell.        
Various forms of interconnect systems are known in the prior art:                A rigid mounting late that incorporates the anode gas and cathode air passages and separator plate function. Contact is made to the cell with contact paste comprising NiO which reduces to Ni in operation on the anode side, and conductive ceramic pastes on the cathode side. These types of interconnects typically are heavy, complex to fabricate, and expensive. There is also very little conformance between parts at operating temperatures, which can induce stress into the cells, leading to fracture resulting in loss of function. In addition, since parts are rigid, there is no allowance for relative motion during thermal cycling which will stress the cell and fracture the contact paste bonds, resulting in poor connection.        A very flexible connecting filament system. In this type of system, no mechanical support is provided for the cell by the interconnect. The cell is allowed to float, which theoretically reduces stress in the cell. Adhesive contact is made between the filaments and the cell with NiO and conductive pastes similar to the above. This type of system results in tremendous variation in the shape and size of the anode fuel gas and cathode air stream passages, which leads to large variations in gas flow, and therefore performance, from one cell to the next. Also, since the cell is actually quite flexible at operating temperatures, it will take shapes that are non-planar. It has been shown, by finite element analysis and in practice, that this can actually result in higher cell stress and a propensity to fracture, particularly during thermal cycling.        A connection system comprising springy formed wire or sheet strips. The intent of this type of interconnect scheme is not only to provide some support for the cell, but also to provide some compliance for surface irregularities and relative motion during thermal cycling. Contact pastes similar to those above are used, although adhesion is not required. A problem with this system is that there will always be slight mismatches in spring forces exerted on the cell, and these mismatches will vary across the cell surface, resulting in localized bending forces and therefore stress in the cell which can lead to fracture. In addition, although the interconnect is compliant, there are still shear forces which can fracture the fragile cathode conductive ceramic paste joint which will still make contact, but at increased electrical resistance.        A thin metal separator plate that has dimples or ribs formed into it for contact with the anode and cathode. Contact pastes as above typically are used. This type of structure is very economical but has several drawbacks. Due to sheet metal forming limitations and the necessary restriction in geometry by using one part, the number, shape, and positions of the contact points are severely compromised. Also, if the plate material is ferritic stainless steel, which is a good choice for low cost and thermal expansion matching to the cell, the plate has very little strength at operating temperatures and the formed-in features will creep, resulting in a reduction of contact and loss in positioning of the cell. If a stronger alloy is used, it will have a thermal expansion mismatch, resulting in severe shear forces that can fracture the contact paste joints, leading to an increase in resistance. Also, there will be mismatched contact forces similar to those in the springy contact system.        
The cathode in a SOFC cell needs to have good electrical contact with the metallic interconnect mesh or other flow feature in the cathode side of the repeating unit. This contact is achieved typically by using a conductive paste but it needs to be low resistance, have compatibility with the cell component, and be durable at the SOFC operating temperatures.
Typical prior art SOFC interconnects have perovskites or silver paste as the contact paste between the cathode and the metal interconnect. Perovskite pastes have a drawback in that they sinter to a brittle ceramic, leading to cracked contact interfaces which lead to higher resistance over time and thermal cycling. Silver paste is a very good conductor and can form a low resistance contact but it has the drawback in that it is volatile at the operating temperature of SOFC (m.p=961° C.) and can cause potential damage to the stack (shorts etc).
What is needed in the art is an interconnect system that provides a mechanically robust joint between itself and adjacent fuel cells that will endure thermal cycling and has minimal electrical contact resistance.
It is a principal object of the present invention to increase the reliability and durability of an SOFC system.